Much effort has been expended in area of ground-based detection of mines. While a number of technologies have been applied to the task, each is particular to the type of mine, the terrain, or the time pressures for detection, i.e. during the stress of combat or during peace time. There is an objective to obtain the highest probability of detection (Pd) and the lowest false alarm rate (FAR). Further, there is the objective cover ground at a high rate of speed which equates to clearance of mines at a faster rate.
Conventional methods include hand-held, ground based vehicle-mounted or airborne detectors. Vehicle-mounted detectors have a high ground speed and hand-held detectors are intended for places that vehicle-mounted detectors cannot reach. The hand-held detectors have a slow ground speed. Airborne detectors are fast but are designed for gross detection of minefields and as such generally have a lower Pd and higher FAR than the other two methods.
The prior art detection apparatus and methodology have employed a variety of different single sensor technologies, including electromagnetic induction, magnetometers, impedance measurements, ground probing radar, millimeter and microwave radiometry, optical and infrared imaging (IR), ultraviolet, acoustics, various nuclear methods, trace gas detection, biodetection, and mechanical probes.
Some hand-held sensors using electromagnetic induction detectors, achieve a desirably high Pd for metallic mines but can be associated with a high FAR, i.e. detecting metallic debris which are not mines. The applicants are not aware of hand-held detectors that can reliably detect nonmetallic mines.
Thermal Neutron Activation (TNA) is another known sensor technology for detection and relative quantification of nitrogen in nitrogen-based explosives. A long standing known application is the use of TNA for detection of explosives in baggage. Applicants are also aware of application of TNA applied to continuously moving mine-detection vehicles for dynamically detecting mines. It is applicant's understanding that this moving application has been unsuccessful thus far due to the incompatibility of covering ground relatively quickly and collecting sufficient gamma rays to reliably assess the nitrogen content of an object.
Prior art attempts have been attempted to assess the data from combinations of IR, radar and metal detectors. "Data Fusion" is known which combines and analyses a plurality of sensor data as a whole. In conventional information-analysis systems, individual data from each sensor is weighted or compared to a threshold, then combined for improving decision-making. One disadvantage with data fusion alone is that despite the potential for improved detection probabilities, there is also a greater likelihood of greater false positive detections.
For ground-based, vehicle-mounted mine detection, the majority of prior art systems are single sensor systems using electromagnetic induction (EMI) or ground penetrating radar (GPR). Known combinations of sensors include using forward-looking IR (FLIR) cameras, EMI arrays and GPR. To date, applicant's believe that data from each sensor has been presented separately to an operator who has had the very difficult task of sorting out the simultaneous information or a combination of sensors merely improves the likelihood of identifying potential hazardous objects
In short, the prior art teaches:
ground-based, vehicle-mounted sensors for detecting mines; PA1 use of multiple mine detecting sensors on one vehicle; PA1 data fusion techniques for combining the data from multiple sensors generally for improving the probability of detection; and PA1 use of massive TNA sensors for baggage interrogation or for detection under moving, short time duration sensing.
There is a demonstrated need for a system which will traverse the ground at a reasonable rate and rather than being able to suggest that the object is likely a mine, a system which will substantially confirming whether a detected object is a mine or not.
Thermal Neutron Activation (TNA) is known for inspecting baggage for the presence of nitrogen, a known constituent of explosives. Generally, a target of interest, such as baggage, is exposed to thermal (slow) neutrons. A reaction between the thermal neutrons and nitrogen-14 causes emission of gamma rays. The emitted gamma rays are captured, counted and processed with electronics to determine the presence or concentration of nitrogen.
To count the gamma rays, events detected by a scintillation crystal are converted to an output signal containing serial pulses. High energy pulses are sought as being indicative of the presence of nitrogen. Low energy pulses are ignored (by applying a threshold or fast discriminator). Unfortunately, as the rate of pulses increases, such as is the result of using a high strength source and a close target, low energy pulses can be too closely spaced in time and pile-up upon each other. The apparent energy of the low energy piled-up pulses can exceed the detection threshold and be incorrectly interpreted as a high energy pulse representing nitrogen. Methods are successfully practiced to reject such pileup at counting rates of upwards of 200,000 counts per second (cps).
Low efficiency detectors such as Germanium (Ge) directly generate pulses having a shape which can be processed with conventional amplifiers at count rates in the 200,000-400,000 cps. To use Germanium detectors would require use of very high strength sources or many detectors--the costs being 10 to 20 time greater than using Sodium Iodide. Further, the Germanium units require cryogenics. High efficiency detectors such as a Sodium Iodide detector (Nal) can use a lower strength source, but the detector collects and output pulses having a shape which is subject to greater incidences of pile-up. Applicant is not aware of amplifiers or methods for rejecting pile-up at count rates from Nal detectors of over 200,000 cps.
Should an object be deemed to be a mine, it needs to be marked for subsequent neutralization, usually by digging it out of the ground. The existing line marking and other spray paint means are substantially mass-less, are difficult to place on ground and are only visible if viewed substantially straight on. Further, paints and the like are usually associated with toxicity and are semi-permanent. There is opportunity and a need for a temporary, environmentally friendly and highly visible marking scheme.